The present invention relates to process measurement techniques and apparatus. Particularly, the invention relates to a measurement technique and apparatus for determining the level of settled particles in a fluid reservoir or process vessel.
In liquid treating plants, it is sometimes desired to measure the level of settled grit or sand in a fluid within a vessel. The level of grit or sand in a liquid within a vessel has been measured typically in two ways: by measuring the distance from the surface of the liquid to the surface of the grit, or by weighing the liquid and grit to determine the percentage of grit by the added weight.
The first method suffers the drawback that, in many process vessels, there is no clear passage from the surface of the liquid to the surface of the grit or sand to make a direct measurement. The second method suffers the drawback that, in a large process facility, the weighing of a large vessel is not always possible or practical.
The present inventors have recognized the desirability of providing an effective method and apparatus for measuring the level of grit or sand in a fluid within a fluid vessel or reservoir. The present inventors have recognized the desirability of providing a method and apparatus for measuring the level of grit or sand in a fluid within a vessel or reservoir that does not require a clear passage down to the surface of the sand or grit or a weighing of the vessel or reservoir to determine to level of sand or grit therein.
The invention provides a method and apparatus for measuring the level of particles, such as sand or grit, in a liquid within a vessel or reservoir. The method and apparatus are simple and cost effectively incorporated into the vessel or reservoir.
Settled particles, such as grit or sand, maintain a submerged level within a vessel by a mechanism wherein the particles support themselves on the bottom of the vessel with individual grains resting on other individual grains. The settled particles result in an angle of repose of the particles. Accordingly, the pressure exerted by the particles is not equal in every direction. The particles do not, in fact, exert pressure in the same sense that a liquid exerts pressure. The particles are supported by the bottom of the vessel, although some lateral force due to the weight is exerted against the side of the vessel. The liquid contained within the vessel surrounds the particles and penetrates the void between the particles. Fluid pressure measured by a conventional pressure measuring device at a level of the vessel below the submerged level of particles is a measure only of the liquid pressure in the vessel. Thus, a static fluid pressure reading at a bottom of the vessel, measured in a conventional manner, will not be useful to determine the level of grit within the vessel.
The present invention provides a technique for using a static fluid pressure reading to determine the level of settled particles within the vessel. The method of the invention includes the steps of:
providing a vessel containing a volume of liquid defining a liquid level, and a volume of particles settled to the bottom of the volume of liquid;
establishing a position within the vessel below the submerged level of particles;
fluidizing the particles above the position;
measuring a fluidized static pressure of the liquid including the fluidized particles; and
comparing the fluidized static pressure with an experimentally established relationship between level of settled particles and fluidized static pressure.
The relationship of fluidized static pressure is proportional to the level of the settled particles within the vessel above the level of the position. The correlation or relationship of the level of settled particles and the fluidized static pressure for the particular liquid and particles is pre-established experimentally.
The fluidizing of the particles can be done continuously or intermittently. A static measurement of the fluid in a non-fluidized region of the vessel, or at the fluidizing inlet before fluidizing, can be used to establish a datum for non-fluidized static pressure with the vessel. This datum is influenced by the density of the liquid and entrained particles within the vessel as well as the liquid level within the vessel. The datum will influence the correlation between fluidized particles and the calculated level of settled particles within the vessel.
The apparatus for performing the method includes a vessel for holding the liquid and the submerged particles, a pipe or line for delivering fluid into the vessel below the submerged level of particles, and a static pressure measuring instrument located at the position. The static pressure measuring instrument is configured to measure static pressure in the vessel at the position. The line for delivering fluid can be from a separate source of pressurized fluid such as pressurized water (city water), or from liquid re-circulated from the vessel. The line can be connected at an inlet end thereof to the vessel at a position above the submerged level of particles, preferably remote from the pressure measuring position. The apparatus can include a pump for delivering the fluid under pressure through the line and into the vessel. The pressure measuring device can be a pressure gauge connected to the line, close to the vessel.
As a further development, the pressure measuring instrument can be a pressure switch that is signal-connected to a solids pump. The solid pump can be operated intermittently as needed to control the level of particles in a vessel, responsive to the pressure measuring instrument.